1. Field of the Invention
The present invention relates to an electric actuator for use in general industrial electric motors, motor vehicles and boats.
2. Description of Related Art
In a relatively small boat in which a propeller is driven by an internal combustion engine, switching between the propeller's forward rotation and reverse rotation is implemented by switching a dog clutch via a wire connected to a lever that is operated by an operator in such a manner as to bring the dog clutch into engagement with a forward gear or a reverse gear. In recent years, however, there have been demands for electric dog clutch switching for the sake of conservation of energy. Here, various types of actuators have been developed for motor vehicle actuators, for example, (refer to Japanese Patent Unexamined Publication JP-A-9-224348), and it is considered that those actuators are used to satisfy the demands.
However, as to actuators for boats, it has to be considered that boats are used in the open sea, and it is desired that boat actuators are developed under a concept which differs from development concepts for general actuators.
For example, in general linear actuators, since they are controlled after an axial displacement is detected in many cases, those linear actuators are used with a limit switch provided in a portion which is displaced in the axial direction. In addition, an electric motor which constitutes a drive source for the actuator is assembled in such a state that the electric motor is exposed on an exterior wall of the housing in many cases.
In the event that a stroke position detection of the linear actuator is attempted to be implemented by a limit switch, however, a forward position and a reverse position of the dog clutch are detected in two positions. In this case, since no intermediate position can be detected, in the event that the dog clutch is stopped between the two positions for some reason, there is caused a problem that the drive circuit cannot determine in which direction the dog clutch is to be shifted.
In addition, in general linear actuators in which electric motors are used as drive sources, in many cases, electric motors are mounted on housings in an exposed fashion for the reason that heat dissipation is promoted. However, the following properties are required for actuators for use in boats.
(1) Water Resistance: In the majority of cases, electric motors, which are electric components, are made inoperable by infiltration of water into the interior thereof. In addition, since it is inevitable that casings of electric motors are made of iron-system metal, there is also caused a problem that the casings are easy to rust. In particular, under an environment in which an electric motor is splashed with sea water, this problem becomes more significant, and a high-function surface treatment needs to be applied to the casing of the electric motor.(2) Weather Resistance: When used outside, since resin or rubber materials are subjected to attack by sunlight or become open to attack by ozone, functions which can resist the attack are required.(3) Oil Resistance: When used in the vicinity of an internal combustion engine, since there is a fear that gear oil, anti-corrosion oil or gasoline is scattered, resistances thereagainst are required.(4) Electromagnetic Wave Resistance Characteristic: When used in a boat equipped with, in particular, a radar system, since there is a fear that the electric motor is exposed to a strong electromagnetic wave environment, a characteristic which can resist thereagainst becomes necessary. An electric motor which meets all the characteristics that have been described heretofore has a problem that the motor is very hard to be mass produced or a production costs thereof becomes very high.(5) Dust Resistance: When used outside, in the event that foreign matters such as dust infiltrate into the interior of the electric motor, due to the foreign matters being bitten between rolling components such as a ball screw and a bearing and meshing components such as gears, there may be caused a problem that the wear of the components involved is promoted significantly or the operating components are locked.
Furthermore, in an actuator in Japanese Patent Unexamined Publication JP-A-2001-280438, the actuator is coupled to a link member via a supporting hole in a distal end of a cylinder which moves in an axial direction. In this case, a load exerted on the cylinder is not necessarily only an axial load, and in many cases the cylinder is subjected to a radial component of force. In addition, there occurs a state in which all the mass of an actuator main body is exerted on the supporting hole. Although this conventional example relates to the actuator for driving a chair or the like, should the actuator be mounted on an outboard engine of a boat, there may occur a case where vibrations of an internal combustion engine are added directly to the actuator, and since the mass of the actuator main body is exerted in the form of a force amplified by vibration acceleration, producing a more stringent load condition.
In the event that a housing in which gears and the like are housed and a cover member which shields the housing are mounted individually on the body of a boat under this condition, a large magnitude of force is applied to the cover member. In particular, in an actuator which is used in a boat, it is an absolute requirement that infiltration of foreign matters and water into a closed space where the gears or the like are housed should strictly be avoided, and consequently, joining of mating surfaces of the housing and the cover member which define the closed space requires high accuracy and a construction which holds the joining of the mating surfaces. The realization of the construction which takes into consideration even a deformation that is caused by a further external force being exerted on the housing and the cover member that constitute the object construction to which such a high accuracy is given entrains difficulty.
In addition, there are a problem of how to lay out wirings to an electric motor and a problem that when applying a sealing agent to the mating surfaces of the actuator with a view to enhancing the water resistance, the sealing agent cannot be applied with good efficiency.
[Second Problem]
In addition, in an actuator disclosed, for example, in the JP-A-9-224348, a nut is coupled to an output shaft thereof, and this output shaft is positioned radially in place by a support member which is fitted in an inside diameter side of a distal end of a cover which is fitted in a housing. In addition, a seal ring is assembled to the vicinity of the support member for preventing the infiltration of foreign matters.
Here, in the actuator shown in the JP-A-9-224348, the support member of the output shaft is formed into a simple tubular shape having an inside diameter which permits the passage of the output shaft therethrough and an outside diameter which enables the fitting thereof in the cover, and the seal ring is annexed to the support member for the purpose of preventing the infiltration of foreign matters into a threaded portion. However, in general, this seal ring is made of rubber materials represented by a nitrile rubber in many cases. The sealing performance of the output shaft by the seal ring is, however, dependent largely on the elasticity of the rubber material.
In the case of the seal ring whose sealing performance has to be dependent on the elasticity of the rubber material in this way, the center of the seal ring needs to accurately be aligned with the center of the output shaft in order for the seal ring to exhibit its sealing function sufficiently. However, since the seal ring itself has generally no aligning function to align the housing with the output shaft, normally, a support member is provided in the vicinity of the seal ring for guiding the output shaft so as to be made to play a role of aligning the center of the seal with the center of the output shaft. Consequently, highly accurate inside diameter dimension, outside diameter dimension and concentricity of the inside and outside diameters, including a fitting gap between the support member itself and the housing, are required for the support member, causing a problem that the production costs are increased.
Furthermore, in providing the support member that has been described above, it has to be considered that no damage be given to the surface of the output shaft by the support member in guiding the output shaft. The reason for this is obvious; a damage given to the surface of the output shaft can be a direct cause for reduction in sealing performance.
Due to these requirements, many support member materials adopt resin members. In particular, where the surface hardness of the output shaft cannot be increased due to various requirements for the actuator (as one of such requirements, in order to impart an anti-rust capability to the output shaft, an SUS material which is not heat treated has to be used as a material for the output shaft), a resin material is generally used. In addition, since the support member has to fulfill the requirement described above, it is natural that no glass fiber, which acts as a dimension stabilizer, can be mixed in a resin material used. This is because there may be a fear that should they be mixed, glass fibers damage the surface of the output shaft.
In mass production of resin support members under the restriction described above, in general, injection molding is used. Although injection molding is suitable for mass production, in injection molding, there is an inherent problem that a sink mark is produced on the surface an injection-molded resin piece when it is set, and the injection-molded resin piece has to suffer from a problem that its dimensions become unstable.
In many cases actuators are used under a highly humid environment. In particular, in the case of actuators being used in boats, the humidity becomes significantly high in the environment where they are used. Consequently, a particularly high sealing performance is required for actuators for use in boats. On the other hand, as has been described above, the support member which is largely involved in the sealing performance needs to be manufactured from resin. However, it is not always true that any resin materials can satisfy the function, and in particular, nylon-based resin materials have a water absorbing characteristic and hence are difficult to be used for this purpose. This is because a material which absorbs water increases the inside diameter of the support member, which increases, in turn, the gap between the output shaft and the support member, leading to a problem that the sealing performance is eventually affected by the increased gap.
[Third Problem]
In an actuator of Japanese Patent Unexamined Publication JP-A-10-201173, a motor, which is a drive source, is constructed to have a centering location relative to a housing, so as to be positioned relative to the housing so that a flange of the motor is brought into abutment with the housing and is then fixed thereto. This motor is disposed parallel to a threaded member, so as to suppress the overall axial length of the actuator.
In the actuator configured according to the related art, the motor, which is the drive source, is exposed to the outside of the housing. Consequently, the motor can be assembled from the outside of a gearbox. In the case of the actuator itself being used in an environmentally stringent location as in an outboard engine of a boat, the motor is preferably configured to be accommodated within the housing. Here, when the motor is accommodated with the housing, it is considered that a bag-like compartment is provided in part of the housing, so that the motor is assembled into the housing from an opening in the housing.
However, in many cases an outer circumferential frame member which covers the motor is formed of sheet metal by pressing, and in many cases the position of a centering location of the motor which positions a rotational axis of a rotational shaft thereof is restricted to an outside diameter portion lying near where a bearing is disposed within the motor. Furthermore, the centering location is in many cases disposed on an output shaft side of the flange, as a boundary, by which the motor is mounted on the housing. According to this configuration, although the motor has to be mounted in the bag-like compartment of the housing from a rear end side of the rotational shaft, this causes a problem that the centering location cannot be secured in the appropriate position.
On the other hand, in the related art actuator that has been described above, a motor side actuator gear is fitted on a distal end of the motor shaft, and a slider side actuator gear is disposed concentrically with the threaded member. A counter side actuator gear is disposed intermediately between the two gears, and a rotational center thereof is disposed on the housing.
When the motor shaft gear and the threaded member gear are coupled by the intermediate counter side actuator gear, there are encountered restrictions such as: normally, the rotational axis must be disposed in a position where it does not interfere with outside diameters of the motor shaft gear and the threaded member gear; the rotational center must be disposed outside a projected shape of the motor in the axial direction; and the rotational center must be disposed in a position where it does not interfere with a screw shaft supporting bearing. Furthermore, a restriction with respect to gear ratio is also imposed from a problem of an inter-gear shaft distance, leading to a problem that the degree of freedom in design is reduced.
In addition, when the motor is disposed within the closed compartment within the housing, there is caused a problem of how to suppress an increase in temperature due to heat dissipated from the motor.
To deal with this problem, in an actuator of Japanese Patent Unexamined Publication JP-A-4-283164, the increase in temperature due to heat dissipated from the motor is suppressed by introducing cool air via a duct which branches from an air conditioning air supply duct of a vehicle. However, in the case of the actuator for use in the outboard engine of the boat, for example, the air conditioning air supply duct itself does not exist on the boat, and hence, a cooling fan needs to be provided separately, leading to a problem that the production costs are increased or the size is enlarged.
Furthermore, in a general linear actuator which uses an electric motor as a drive source, in many cases the electric motor is mounted on a housing in an exposed fashion for the reason that heat dissipation is promoted. Consequently, heat generated from the electric motor tends to be confined within the housing, and this anticipates troubles that would be caused by the confined heat. In the event that the capacity of the motor is reduced to deal with this, the heating value thereof can be reduced to some extent. However, since an output torque is reduced in accordance with the reduction in the heat value, in order to provide sufficient power, the reduction gear ratio has to be increased, this making difficult the realization of a compact actuator.